understanding mechanisms for design of advanced superconductors

Workshop on Superconductivity May 8-11, 2006Workshop on Superconductivity May 8-11, 2006Basic Energy SciencesBasic Energy Sciences

Understanding Mechanisms of Superconductivity and Design of Advanced Superconductors

Warren E. Pickett (UCDavis)

Basic Research Needs for Superconductivity

Based in part on 2006 DOE/BES Report

Outline:• history, mechanisms of HTS• perspective: requirements of a theory of HTS• outstanding challenges in mechanisms (non-HTS) stimulated by new materials discoveries• design of new, advanced superconductors

Basic Energy SciencesBasic Energy Sciences BES Report on Basic Research Needs for Superconductivityhttp://www.sc.doe.gov/bes/reports/abstracts.html#SC

BCSTheory

50th anniversary of the BCS paper1360 citations as of 2003 5th most of any in PR/PRX/PRL/RMP

John BardeenLeon CooperJ. Robert Schrieffer

Phys. Rev. 108, 1175-1204 (1957)

S. Redner, Physics Today, 2005

In the beginning…..


the superconductor tsumani(late 1986)

Nobel Prize in Physics, 1987

30 K onset

20th Anniversary


HTS SuperconductivitySession B1 (yesterday): 20th anniversary of High Tc Superconductivity 'Woodstock Session’

54 sessions at this meeting with “supercond” in the title

This continues a 20 year tradition of numerous superconductivity sessions at the APS March Meeting.

6th anniversary of MgB2 mini-Woodstock


DOE Workshop, May 2006

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Publication activity in HTS remains prodigious

Chaos

Euphoria

Nitty-gritty

It is essential tosustain theprogress in HTSand the associatedfundamental understanding and materials expertise thatthat is accumulating


20th Anniversary of High Tc

Science Nov 2006

NatureMarch 2006

Synopsis: elaboration and acceptance of the mechanism of HTS mechanism is not imminent


Proposed Mechanisms of HTS Superconductivity

[from D. J. Scalapino, gleaned from presentationsat M2S-HTS, Dresden, July 2006]

Jahn-Teller bipolaronsstripes (role of inhomogeneities)RVB-Gutzwiller projected BCSelectron-phonon + Uspin fluctuationscharge fluctuationselectric quadrupole fluctuationsloop current fluctuationsd-DW, d-CDWquantum critical point fluctuationscompeting phasesPomeranchuk instabilitiesd-to-d electronic excitations

DJS: there is plenty of data available to decide between mechansims

Possibility: there is too much data to decide between mechanisms

Is “mechanism” the question…?


A Faithful and Convincing Mechanism of HTS

Faithful theory * (semi)quantitative explanation of the observations that are central to optimally doped HTS (focus!) * no spurious predictions

Convincing theory * majority of workers in the field accept the theory * there are no seriously competing theories * no `reasonably objective’ person can disbelieve its general applicability

I.e. “BCS-like in its convincibility.” Is this a plausible goal?

What is needed to constitute ……..


A Faithful and Convincing Theory of HTS

In principle, to discover the mechanism * focus on optimally doped region * analogy: mag. impurities in BCS sc’or

In practice: entire phase diagram needsto be understood

* majority of workers seem to accept this * this is a much broader goal than `the mechanism’, it is `the theory’

What is needed to constitute ……..

Complication: thereare other similarphase diagramsin low-Tc systems



First address the broadest issues

SAMENESS: why layered cuprates and only cuprates?* all HTS have CuO2 planes; no others are HTS* there are other quasi-2D doped insulating antiferromagnets;

why only cuprates? VARIATION: why so much; what is the essence; what does it tell us?

What are the broadest issues for ……

Hg2223



Big issue: what distinctions need to be explained? Some propositions:

Shape of Fermi surface (system dependent); effect on mechanism Value of Tc (within factor of two, with correct trends) Symmetry of superconducting order parameter Low E excitations: 1-particle; magnetic; phononic; other collective Inhomogeneities: patterns, connections to other phenomena Trend of Tc in cuprate classes: [Bi] < [Tl] < [Hg] Trend of Tc with number of CuO2 layers (maximum at 3 layers) Pressure dependence of Tc: theory must work at any volume (many, many more related to the entire HTS phase diagram)

What may be needed to comprise ……

Several proposed mechanisms unify certain aspects of HTS

Theory of the entire phase diagram is a huge issue (an attractive one)


Additional Developments in hTS Materials [hTS == unexpectedly high Tc]

40: MgB2 40: Alkali-doped fullerenes 35: (Ba,K)BiO3 [BKBO] (discovered in 1986) 25: Alkali-doped HfNCl, ZrNCl 25: Elemental metals under pressure 19: PuCoGa5 a novel heavy fermion sc’or 18: Y2C3 -- who ordered this one?

2D triangular lattice oxides & chalcogenides

More on Mechanisms+Materials


J. M. An and WEP, Phys. Rev. Lett. (2001)J. Kortus et al., Phys. Rev. Lett. (2001)Y. Kong et al., Phys. Rev. B (2001)K.-P. Bohnen et al., Phys. Rev. Lett. (2001)……..more…….

1. MgB2: covalent bonds become metallic2. Deformation potential D=13 eV/A (amazingly large, especially for a metal)3. 2D (cylinder) Fermi surfaces focus strength4. Yet structure remains stable: intrinsic covalency

T. Yildirim (NIST)

MgB2 is the champ (Akimitsu group, 2001)


Yttrium Sesquicarbide Y2C3

Coupling to high frequencies?

Simple cubic Bravais lattice of Y8C12 primitive cellsDistinctive feature: triply-bonded C2 dimers

Tc = 18 K (Akimitsu group)

Singh & Mazin, 2004 C2 dimer state near EF Ag modes: 120 K, 1000 K

Coupling to hard C2 mode may be important for the `high’ Tc

[Tc(La2C3) = 11 K]


Pressure as a Tool to Produce Superconductors: Elemental Metals under Pressure: Tc=20-25K

Nesting function in three planes

Li: Tc up to 20 K Y: Tc up to 20 K

Ca: Tc up to 25 K !

Lithiumfcc Li: strong coupling, phonon anomalies, instabilities under pressure

Eliashberg theory: UCDavisDFT for superconductors: Berlin, Gross et al.


Observations about Carrier-dopedLayered Transition Metal `Oxides’

Electron-doped TaS2 Hole-doped LiNbO2 Hole-doped NaCoO2 (hydrated) Electron-doped TiSe2

Observation about Carrier-dopedLayered Transition Metal Nitride

Electron-doped ZrNCl, HfNCl


Co-Intercalated Layered Dichalcogenides (D. C. Johnston et al., 1983-4)

1/3 2 2Na TaS yH O!

2

3

9/1TaSY

+

2

3

9/1TaSLa

+

2

2

6/1TaSMn

+

3

23+

! Ta

Several distinct phases y=0, 2/3, 0.8, 3/2, 2

All have Tc = 4-5 K


Li1-xNbO2: 5 years after HTS (Stacy group, 1991) Tc = 5.5 K

Layered TM oxideTrigonal-prismatic coordinationTriangular latticeNb d1+x configurationSingle d(z2) band is occupiedHole-doped from semiconductor

Single-band triangular lattice systemSuperconducting in a wide range

around x ~ 0.5


Na1-xCoO2, the Dehydrated Superconductor [add water!]

Edge-sharingCoO6octahedra

Jorgensen et al. (2003) Takada et al., Nature 422, 53 (2003);

Adv. Mater. 16, 1901 (2004)

Tc = 4.5 KTriangular lattice Hole-doped from Co3+ semiconductorOctahedral CoO6

Na1-xCoO2∗yH2OSuperconductingaround x ~ 0.3


CuxTiSe2: CDW--> Superconductivity

Layered 2D TM chalcogenide Triangular lattice system Trigonal-prismatic coordination CDW has long been studied Nominal d0 Ti configuration Electron-doped --> sc’y

Morosan et al. (2005)

Maximum Tc=4.2K at x=0.08


Synopsis: Tc in 2D Triangular Oxides/Chalcogenides

AxTaS2

Li1-xNbO2 NaxCoO2*yH2O

CuxTiSe2All Tcs hover around 5 K


Alkali-doped AxZrNCl (15 K) & AxHfNCl (25 K)

Double Zr-N layer, corrugated graphiticStrongly 2D bandsElectron-dopedInverse ‘isotope shift’

Structure is somewhat MgB2-like; so is it electron-phonon?

Heid & Bohnen (2006) el-ph coupling strength is not large enough

In-plane d band holdsthe carriers

Superconductor-insulator transition at x=0.06

Bill et al. (2003) Coupling to/ screening by low energy plasmons may be important


PuCoGa5: 18.5 K (Sarrao et al. 2002)

Order of magnitude higher than previous heavy fermion sc’y

Other heavy fermion superconductors: Tc < 2 KPuCoGa5 may provide the key to HF sc’y mechanism


Design of new superconductors: is it viable?

Rational Design/Searchfor new hTS

Example of one designcriterion, enabled byunderstanding of mechanism

Select band structureto enable the phononsto use more of theBrillouin zone


Database driven design/search

……..imposing phase stability.Example: Design of Li2B2 (“MgB2”). Considered several structures. Checked stability. Calc’d phonons.

Kolmogorov, Curtarolo, Calandra cond-mat/0603304, 0701199

Rational Design/Search for new hTS

Tc ~ 15 K calculated


Pasteur’s Science

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understanding mechanisms for design of advanced superconductors

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